Connectors are used widely in electronics to interconnect microelectronic devices such as semiconductor integrated circuits (ICs), printed wiring boards, system boards, backplanes, and cables of various sorts. A socket is a type of connector used to connect terminals on an electronic device to corresponding contacts on a printed circuit board or other electrical interconnection apparatus. Sockets are often arranged in an array of “female-type” elements that are intended to engage “male-type” elements of a terminal array. In addition, sockets are routinely used in systems for: (a) testing electronic device performance (an assortment of types of sockets has been developed to connect to a device under test (“DUT”) having a wide variety of terminals and configurations); or (b) burn-in of electronic devices at elevated temperatures. A cable connector is typically used to connect an array of terminals on an electrical cable to a group of corresponding electrical terminals or other conductors. Backplane connectors and inter-board connectors are typically used to connect an array of terminals on one printed wiring board to a corresponding array of terminals on another printed wiring board.
Advances in the density and speed of electronic devices are placing additional demands on connectors. In particular, a continuing increase in the wiring density of electronic systems requires a corresponding advance in the density of connectors as determined by the number of contacts per unit area. Further, at higher frequencies and clock speeds, the size and the self inductance of connectors are becoming an important limitation to system performance. In addition to a lower inductance, advances in impedance control and shielding are required for future electronic systems.
Prior art connectors are differentiated typically according to contactor type and intended use (i.e., application). As such, connectors used in applications in sockets are typically specifically designed to make electrical contact to microelectronic devices having specific types of device terminals. For example, specific types of device terminals contacted by sockets include pin grid arrays (“PGAs”), J-leads, gull-wing leads, dual in-line (“DIP”) leads, ball grid arrays (“BGAs”), column grid arrays (“CGAs”), flat metal pads, land grid arrays (“LGAs”), and many others. Many contactor technologies have been developed to provide sockets for microelectronic devices having this variety of terminals.
In addition to the foregoing, further differentiation among prior art sockets refers to low insertion force (“LIF”) sockets, zero insertion force (“ZIF”) sockets, auto-load sockets, burn-in sockets, high performance test sockets, and production sockets (i.e., sockets for use in products). In further addition to the foregoing, low cost prior art sockets for burn-in and product applications typically incorporate contactors of stamped and formed springs that are adapted to connect to terminals on a DUT. In still further addition to the foregoing, for high pin-count, prior art sockets, a cam is often used to urge device terminals laterally against corresponding contactors to make good contact to each terminal while allowing low or zero insertion force.
For specialized applications, prior art sockets have used a wide variety of contactors, including anisotropic conductive sheets, flat springs, lithographically formed springs, fuzz buttons (available from Cinch, Inc. of Lombard, Ill.), spring wires, barrel connectors, and spring forks, among others. Prior art sockets intended for applications where many test mating cycles (also referred to as socket mount-demount cycles) are required typically use spring pin contactors (also referred to as spring probes or spring contacts) of the type exemplified by Pogo® spring contacts (available from Everett Charles Technologies of Pomona, Calif.).
Spring probes for applications in the electronics test industry are available in many configurations, including simple pins and coaxially grounded pins. Most prior art spring probes consist of a helical wire spring disposed between a top post (for contacting terminals on a DUT) and a bottom post (for contacting contacts on a circuit board—a device under test board or “DUT board”).
Prior art sockets typically have a plurality of contactors disposed in an array of apertures formed through a dielectric holder. By way of example, a high performance, prior art test socket may incorporate a plurality of Pogo® spring contacts, each of which is held in a pin holder with an array of holes through a thin dielectric plate. The dielectric material in a high performance, prior art test socket is typically selected from a group of dimensionally stable polymer materials including: glass reinforced Torlon 5530 available from Quadrant Engineering Plastic Products, Inc. of Reading, Pa.; Vespel; Ultem 2000 available from GE Company GE Plastics of Pittsfield, Mass.; PEEK; liquid crystal polymer; and others. Individual Pogo® spring contacts are typically selected and designed for signal conduction at an impedance level of approximately fifty (50) ohms. In certain high performance, prior art configurations, the contactor is a coaxial type contactor having a center spring pin with a contactor barrel body enclosed within a cylindrical, coaxial, ground shield spaced to achieve a desired signal impedance, typically fifty (50) ohms.
Connectors used in applications for connecting one printed wiring board to another printed wiring board can be classified by type, including edge connectors, pin-in-barrel connectors, stamped spring connectors, spring fork connectors, LAN-grid array connectors, conductive elastomeric connectors, and various other types known in the art.
Cable connectors adapted to flat cables are generally similar to printed wiring board to printed wiring board connectors with an added feature that one side of a connection is made to a flex cable or a flat array of wires rather than to a printed wiring board. Cable connectors adapted to a round bundle of wires are generally of the type employing a pin in barrel wherein a spring in the barrel retains the pin and applies a lateral force on the pin to establish reliable electrical contact. The spring incorporated into the barrel element may be a spring insert, a bundle of spring wires or an integral spring in the barrel.
The class of connectors used for socketing ICs is specialized and important in the electronics industry. The recent growth in use of BGA terminals for IC packaging has resulted in use of new and varied sockets adapted to BGA terminals for increasing terminal count and area density. BGA sockets have evolved in several directions. One type of BGA socket involves use of a cam driven spring wire to contact the side of each ball. In another type of BGA socket, spring pins or Pogo® pins have been adapted for use in such BGA sockets for certain applications in which the high cost of a socket is acceptable.
Low-cost BGA sockets for mass market applications have evolved the use of stamped and formed springs that cradle each ball of the BGA and provide some measure of mechanical compliance needed to urge a spring connector into contact with a mating ball. In such sockets, variations of stamped and formed springs are configured to use two or more formed springs to grip each ball, and thereby, to make positive electrical contact while retaining the ball mechanically. However, miniaturization and density of mechanically stamped and formed springs are limited to a certain size by present manufacturing capabilities. Although advances continue to be made in the manufacturing art of stamping and forming springs, sockets with contactors thusly made are limited in density by the complexity of stamping and forming very small miniaturized springs. Further, the mechanical compliance of a stamped and formed spring is typically small in a vertical direction, i.e., perpendicular to a substrate of a ball. Because of small compliance in the vertical direction, a miniature stamped and formed spring may be unable to accommodate motion of a contactor support relative to a ball mated to it, thereby allowing vibration, mechanical shock load and forces, flexure, and the like to cause the connector to slide over the surface of the ball. It is known in the industry that repeated microscopic motion of one contact relative to a mating contact causes fritting or a build up of small particle debris that can lead to contact failure.
Stamped and formed spring contacts are typically held in an array of shaped through holes in a molded plastic housing to form a connector assembly. As connector assemblies are miniaturized, the molding and assembly process become increasingly difficult and costly, thereby limiting the extension of connectors based on formed spring contacts to higher densities.
BGA sockets have also been constructed with contactors that make electrical contact to a bottom region of a ball by means of bundles of helical wires, wires in elastomeric material, cantilever springs, lithographically formed flat springs and other contactors that apply force vertically to a mating ball. The vertical force is necessary to make a good connection between a ball of a BGA and such force is significant for BGA packages with a large number of balls or bumps. For example, the clamping force for a BGA socket that applies force vertically to 1200 contact bumps is as high as 30 Kg to achieve adequate contact to each of the contact bumps. The clamping force needed by BGA sockets that make contact by applying force vertically is an increasing problem as the number of contact bumps increases into the thousands.
As is well known to those of ordinary skill in the art, a primary function of prior art connectors is to provide reliable and repeatable electrical contact to electrical terminals without causing damage to either. Further, a connector must provide a low resistance connection over a product lifetime that involves repeated temperature cycles, mechanical shock, vibration and flexure. As such, contact resistance is one measure of reliability of a connector as determined as a function of a number of temperature cycles, a number of drops, a number of flexures and a G-force level of vibration. As the size and spacing of terminals on microelectronic devices continue to be miniaturized, maintaining contact between the terminals and socket contactors is proving increasingly difficult. Sockets that rely upon frictional forces to retain the microelectronic device in the socket cannot easily be extended to terminal densities projected for the future. Improvements are needed in technology for contacting and retaining microelectronic devices in sockets and connectors of future electronic systems.